2
$\begingroup$

Imagine you are placed in an unknown location in the middle of the ocean. You see the moon at its half.

Which are the different "sets of information" which, if provided to you, would allow you to tell correctly if the moon is waxing or waning?

As you might know, there are several different, independent methods to answer the above based on different space-time information. From my rudimentary/amateur knowledge of astronomy I have identified three. I am, however, curious of all the alternatives.

I am particularly interested in two scenarios:

  1. You cannot see the landscape of the surface of the moon (say, a thin layer of clouds cover it).
  2. You can see the landscape of the surface of the moon (i.e. you can see half of the "moon rabbit")
$\endgroup$
8
  • $\begingroup$ You mean besides the obvious bit of what the moon looks like? Kind of seems like homework... $\endgroup$
    – Jon Custer
    Oct 19 '18 at 15:32
  • $\begingroup$ @JonCuster Definitely not homework, trust me. Just curiosity. What do you mean by "what the moon looks like"? That probably refers to point 2. But what about point 1, where you do not see how the moon looks like? $\endgroup$
    – luchonacho
    Oct 19 '18 at 15:35
  • 2
    $\begingroup$ Even with a thin layer of clouds, the moon looks different between waxing and waning. Consider where the moon and the sun are during the two, and what bits of the moon you will see lit. $\endgroup$
    – Jon Custer
    Oct 19 '18 at 15:36
  • $\begingroup$ @JonCuster If it's nighttime, you do not have the sun as a help. Also, if you are in north hemisphere and look south, position is contrary to that if you are in the south hemisphere, where you look north. In principle you don't know where you are. $\endgroup$
    – luchonacho
    Oct 19 '18 at 15:44
  • 1
    $\begingroup$ It rises to the east and sets to the west. You know E/W, so now the only question is right/left. Sadly, most people don’t spend a lot of time outdoors away from light pollution these days. $\endgroup$
    – Jon Custer
    Oct 19 '18 at 15:52
4
$\begingroup$

I can't provide a comprehensive answer of all the different ways you could tell, but one method would be to 1. Look for Polaris. 2. Observe which side of the moon is lit. If you can see Polaris (the North Star), then you are in the Northern Hemisphere. In that case, if the right side of the moon is being lit, then it is waxing, and if the left side is being lit it is waning. Reverse the "left <-> right" relation if you can't see Polaris and are therefore in the southern hemisphere.

$\endgroup$
2
  • $\begingroup$ Thanks! By left/right side you mean wrt standing facing the equator? Also, if you cannot see Polaris, then you could not know in that precise instant? $\endgroup$
    – luchonacho
    Oct 19 '18 at 16:18
  • $\begingroup$ With regards to you looking at the moon. Unless the moon is directly overhead, you should be able to distinguish between the left or right side of it being bright (assuming you don't stand on your head to look at the moon as well). I suppose if you were close to the equator (roughly within the tropics +- 5 degrees) you would have to do a bit more calculation with Polaris to get your exact latitude, and then you would need some more information for where the moon is in its orbit to tell for sure. $\endgroup$
    – enumaris
    Oct 19 '18 at 16:24
2
$\begingroup$

Time of day (or time of moonrise) and location of the moon give information about the phase.

The phase of the moon depends on the angle between the moon, the earth, and the sun. With respect to the stars and the sun, the moon appears to move from the western sky to the eastern sky as it orbits.

This means that after new moon (a waxing moon), it will rise sometime between sunrise and sunset. A full moon will rise very close to sunset. A waning moon will rise between sunset and sunrise.

If its midnight and the moon is visible, then you may not know exactly when it rose. But if it is obviously in the eastern (recently risen/waning) or western sky (rose many hours ago/waxing), then you have some info on the phase.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.